The hypothesis to be tested by the proposed research is that chloride currents are an important ionic determinant of cardiac rhythm. Activation of chloride currents may have significant effects on transmembrane potential, including depolarization of the resting membrane and acceleration of repolarization. Three seemingly distinct cardiac chloride currents can be activated by elevated intracellular calcium, enhanced sympathetic tone and cell swelling. In order to judge the role of chloride currents as an ionic determinant of cardiac rhythm, we must first understand the distribution, pharmacological and biophysical properties of cardiac chloride currents. Single cell voltage clamp studies will be conducted on dog and human cardiac myocytes. Human atrium and failing human ventricle have only the swelling-induced chloride current. Non-failing human ventricle will be examined to determine if chloride currents differ from failing human ventricle. If not, studies will concentrate exclusively on the swelling- induced chloride current. Biophysical properties that will be studied include rectification properties, ionic selectivity, permeability and conductance, voltage- and time-dependence, activation mechanism and single channel properties. The direction of chloride current flow and the rectification properties of the current can depend on intracellular chloride activity. Consequently it will also be important to improve our understanding of the normal regulation of intracellular chloride activity and changes in intracellular chloride activity during experimental manipulations. To this end, single cell studies will be conducted using chloride-sensitive patch electrodes. To evaluate if chloride currents contribute to cardiac electrical activity the effect of cell swelling on transmembrane potential will be examined to see if action potential changes attributable to chloride current activation can be identified. The sensitivity of action potential changes to chloride substitution and chloride channel blockers will be studied. The proposed studies will provide the information that is needed to determine the role of chloride currents in cardiac rhythm. Since swelling is associated with ischemia, the effect of cardiac chloride currents is likely to be enhanced under pathological conditions. Therefore the use of chloride current modulation as an antiarrhythmic intervention may provide improved specificity over presently available agents that block cation currents.